Effects of spray application on the control of fungal diseases in top fruit

Abstract: The progress that has been made in the past two decades has significantly improvedapple scab control, especially during the primary season. This may easily be observed on organicfruit farms which today reach a level of control at least as good as IP-farms do. Despite thisprogress at the end of the growing season at least in more humid climates (e.g. Lake Constancearea) in both growing systems the percentage of highly infected orchards of susceptible varietieslike “Jonagold” statistically is increasing since many years. Since this increase is occurringdespite intensive spray programmes with classical protectant fungicides it may be interesting toassess for a possible effect of spray application which besides secondary inoculum, shoot growthand physical infection conditions is a key parameter influencing the epidemics of apple scab inthe secondary season. Based on a canopy related dosing model, the effects of canopy adaptedwater volume, forward speed and fan power on spray deposits have been compared with commondosing and application rules using fixed water volume, preset forward speeds and nominal fanpower. Spray cover from three canopy systems has been analysed on deposits, coverage anddroplet deposit density. The canopy related application could almost compensate a reduction ofwater volume per ha as canopy width decreased, leading to very similar coverage and dropletdeposit density on the upper leaf surface and improved spray cover in the centre of broadcanopies. On the lower leaf surface both methods resulted in a strong oversupply increasing ascanopy width increased. Despite improved application efficiency also a canopy related sprayapplication could not reduce the number of leaves with average deposit densities belowapproximately 16cm-2 supposed to be the minimum droplet deposit density for captan. The UVphotographsalso disclosed a small part of the leaves with irregular spots that were not or justpoorly covered because leaves or parts of them where shielded by other leaves, shoots and fruit,but apparently also have been improperly exposed to the spray mist. If this is assumed to be thesame on young and susceptible leaves, a percentage of the leaf area remains insufficientlyprotected, allowing a small fraction of the landed spores to germinate and infect. The risk thatone or more spores land in such a gap and infect, is increasing as the number of spores availableper unit area is increasing. This effect is supposed not to be a question of dosing and watervolume rate but is more related to the random orientation of the individual leaf and itsneighbouring tree structures. Therefore the most effective solution is an early termination ofshoot growth to prevent the development of susceptible tissue as early as possible.